Two to three percent of babies have some type of birth defect. Down syndrome is the most common chromosomal abnormality to cause mental retardation. Genetic screening and diagnostic tests can help you find out if your baby is at risk.

Genetic Screening
There are two types of prenatal testing used to detect chromosomal abnormalities: screening and diagnostic. “Screening tests do not indicate that disease is either present or absent, but tell us whether the chance of disease is higher or lower than otherwise expected for an individual patient,” says board-certified Ob/Gyn Steven C. Goldberg, M.D., FACOG. “The purpose of genetic screening is to assist a pregnant woman in evaluating the need to have additional diagnostic testing.” Physicians test to find possible genetic diseases in unborn babies and to find out if people are carrying a gene for a disease their children may inherit. “Screening tests are non-invasive and pose no threat to a woman’s pregnancy,” says Dr. Goldberg.

Types of screening tests include:

Between 11 – 14 weeks, a nuchal translucency ultrasound (NT) can determine a woman’s risk of having a baby with Down syndrome and also detect fetal cardiac abnormalities. This screening measures the fluid under the skin at the back of the baby’s neck. All babies have some fluid, but in many babies with Down syndrome, the nuchal translucency is increased. Done alone, this test picks up about 75 percent of babies with Down syndrome, and when combined with a maternal blood test, the detection rate improves to about 90 percent and also detects other potential abnormalities.

Maternal Serum Screen
(double screen, triple screen or quadruple screen)
Between 15 and 18 weeks, MSS screening looks for “markers” for Down syndrome in the mother’s blood. A double screen looks for two markers; a triple screen looks for three, a quadruple screen for four. The double test detects about 59 percent of Down syndrome babies, and the quadruple about 75 percent. MSS testing doesn’t actually show if a baby has a chromosomal problem, but instead gives an estimate of risk. Only two or three out of every 100 women who have an abnormal MSS will have a child with birth defects. These tests can also indicate the possibility of abnormalities such as spina bifida.

Diagnostic Testing
“Diagnostic testing, such as amniocentesis or placental biopsy, are offered to pregnant women whose screenings indicate a higher risk of having a baby with some type of abnormality,” says Dr. Goldberg. “These tests are more invasive and may bring a slight risk to the pregnancy, but they will determine whether the condition you are screened for is actually present.”

A woman can then evaluate her options for her pregnancy and parents can optimize plans for the baby’s birth and care at the time of delivery. “An advantage to knowing in advance is to have time to plan and get prepared,” says Dr. Goldberg. “Once you know your risks, it might help to speak with a genetic counselor or a maternal-fetal specialist to help you decide the best course of action for you and your family,” concludes Dr. Goldberg.